Extreme ultraviolet (EUV) lithography, which is based upon exposure with the portion of the electromagnetic spectrum having a wavelength of 10-20 nanometers, may be used to print features with smaller critical dimension (CD) than other more conventional optical techniques, such as those utilizing deep ultraviolet (DUV) radiation or so-called 193 nm lithography.
The mask utilized for the extreme ultraviolet lithography is a sophisticated unit including a Bragg mirror or dielectric mirror deposited on a substrate, which is relatively expensive due to the low tolerances for any defects and high required uniformity of these masks. These masks require frequent cleaning to reduce or eliminate defects during the optical lithography operation. The cleaning is typically performed at an elevated temperature to enable and/or enhance the efficiency of the cleaning chemistry. In addition, during use the masks are inadvertedly heated through exposure with extreme ultraviolet light. Thus, the mask is frequently exposed to temperatures above ambient during the mask's lifecycle and is used at temperatures exceeding ambient during normal operation. Consequently, these conditions can cause several types of chemical diffusion and chemical reactions within the multilayer stack of the Bragg mirror. For example, surface organics may diffuse into the stack. In addition, the reflective and spacer layers within the stack can start chemically intermixing and reacting at their interfaces. Over time, the performance degrades for the mask in the mask will eventually fail due to loss of its carefully optimized optical properties.
It is within this context that the current embodiments arise.